The present invention relates to a cellular system, in which mobile stations communicate with one another via base stations located in respective service areas or cells. More particularly, the present invention concerns a method of transmission power control in a cellular system, which provides hand-over control permitting continuous communication of a mobile station and also transmission power control when the mobile station is moving between two base stations.
Some CDMA (code division multiplexed access cellular system) adopts a technique called soft hand-over system, inter-cell lines are switched when a mobile station moving between cells approaches the borderline between the cells, while continuing simultaneous communications with a plurality of base stations. This technique is detailed in U.S. Pat. No. 5,101,501 (Gilhousen et al., "Method and System for Providing a Soft Handoff in Communications in a CDMA Cellular Telephone System", Mar. 31, 1992).
In the CDMA system, the same frequency is used by a plurality of lines, and the reception power (or desired wave power) of a signal on a certain line constitutes interference wave power to other lines. Therefore, in an up line, via which the signal is transmitted by a mobile station and received by a base station, increase of the desired wave power beyond a predetermined level results in an interference power increase, thus reducing the line capacity. To prevent this, the transmission power on the up line of the mobile station should be stringently controlled, In such an up line transmission power control, the base station measures the desired wave power and, when the desired wave power is high, it sends out an up line transmission power reduction control command to the mobile station, while sending out an up line transmission power increase control command to the mobile station when the desired wave power is low. This method of transmission power control is detailed in U.S. Pat. No. 5,056,109 (Gilhousen et al, "Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Mobile Telephone System", Oct. 8, 1991).
In up line transmission power control during execution of the soft handover, a plurality of base stations measure the desired wave power from a mobile station, and send independent up line transmission power control commands to the mobile station. When the mobile station receives different control commands, it preferentially follows a transmission power reduction control command. This method is described in TIA/EIA Interim Standard, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" and TIA/EIA/95-A, Telecommunications Industry Association, May 1995, "6.6.6.2.7.2 Reverse Traffic Channel Power Control during Soft Handoff". As shown, when control commands from base stations are different, a transmission power reduction control command is preferentially followed, thus preventing the surpassing of a desired value of control by the desired wave power in any base station and realizing high line capacity of the up line. In this method, it is important to reduce the probability of failure of the transmission power reduction control signal reception by the mobile station.
In the down line, via which the signal is transmitted from the base station and received by the mobile station, a high line capacity is realized by transmission power control such that the ratio between the desired wave power and the interference wave power is a predetermined value. In the up line transmission power control, an up line transmission power control command is transmitted to the mobile station to the mobile station by utilizing the down line as described before, and during the soft handover execution the down line transmission power should be controlled such that the mobile station can receive up line transmission power control commands sent out from a plurality of base stations.
A conceivable method to this end is to control the down line transmission power such that the desired power levels from the base stations are equal at the mobile station. In this method, base stations with higher transmission losses up to the mobile station set correspondingly increased transmission power. Increasing the transmission power, however, increases the interference wave power and reduces the down line capacity. The down line capacity reduction can be suppressed by controlling the transmission power levels of the base stations to be equal. This method is described in Anderson, "Tuning the Micro Diversity Performance in a DS-CDMA System", Proc. IEEE 44th Vehicular Technology Conference, pp. 41-45, June 1994.
In the above prior art transmission power control method, however, the desired wave power sent from a base station with a high transmission loss to the mobile station is low. In this case, the probability of failure of the up line transmission power control command from the base station and also the probability of failure of the up line transmission power reduction control command by the mobile station are increased.
Instantaneous transmission loss variations vary with the frequency. In a cellular system in which different frequencies are used for the up and down lines, therefore, the transmission losses in the up and down lines are different. As a result, due to the low up line transmission loss when the transmission power reduction control command is sent with the desired control value at the base station, a case may arise that due to a high transmission loss in the down line the desired wave power in the mobile station is low, resulting in a failure of the transmission power reduction control command by the mobile station.
When the control commands from all the other base stations are transmission power increase control commands at this time, the mobile station increases the transmission power. In consequence, the desired wave power becomes excessive in the base station, resulting in increase of the interference wave power and reduction of the up line capacity of that base station compared to the case when the transmission control power is executed without control command reception failure. In such a transmission power control method, in which the reception power levels from the base stations are made equal, the probability of receiving all up line transmission power control commands is reduced, giving rise to a problem of reducing the up line capacity.